A telephone line monitor supervises a variety of conditions-such as an off-hook condition in which the telephone handset is lifted in preparation to placing a call, dial pulsing in which the telephone line is repeatedly shifted between the on-hook and off-hook conditions by the telephone dial mechanism, and "tapping" in which the hook switch is momentarily depressed by the user in order to signal the telephone switching machine.
All of the aforementioned conditions are characterized by changes in condition of the telephone line from an "on-hook" condition to an "off-hook" condition. Once these changes are detected, conventional telephone circuitry can distinguish between an off hook condition, dial pulses and tapping by determining the duration and pattern of the changes. The results are then used to obtain a variety of information. For example, by monitoring the generation of dial pulses, it is possible to record called numbers, call dates and times.
The change between the on-hook and off-hook condition is generated at the telephone handset. During normal operation the switching circuitry to which the handset is connected places a voltage on each telephone line. When the handset is "on-hook" it appears as a high-impedance across the telephone line, and accordingly, the switching circuit voltage appears across the telephone line as the "on hook" voltage.
When a telephone line goes "off-hook" it places a low impedance across the telephone line. This low impedance reduces the on-hook line voltage to a lower "off-hook" voltage value and also allows a significant increase in the line current.
Conventional telephone line monitors generally detect the change of current in the telephone line between the on and off hook conditions because this change of current can be reliably detected regardless of the actual line voltage. Accordingly, such telephone line monitors must be connected in series between the telephone line and the telephone set. A conventional connection is shown in FIG. 5 in which line monitors 3 are connected in series with line 1 and telephone sets 2.
This required series connection is inconvenient in a key telephone or PBX system such as shown in FIG. 5 in which PBX 5 has a line, 1, to which several telephones 2 are connected by means of plugs and jacks shown schematically as outlines 4. In such a system conventional telephone line monitors 3 must be connected in series with each of telephone sets 2 as shown in FIG. 5. Thus, the number of telephone line monitors required equals the number of telephones in the system, even if the number of telephone lines is much lower.
Further, in a modern telepone system which uses plugs and jacks, the series connection is difficult to retrofit to existing systems. Since the conventional monitor must be inserted into each line, in an existing system the monitor must be placed at each jack location. For example, a typical connection would involve inserting a plug of the telephone line monitor into the wall jack and then inserting the telephone set plug into a jack provided on the telephone line monitor. While this connection is relatively simple, it requires placing the line monitor circuitry at or near the actual location of the telephone and, thus, leaves the monitor circuitry exposed to the user where it is likely to be damaged or tampered with. In addition, since the line monitors are remotely located at each tel phone position, there is generally no easy way to forward the detected signals back to the PBX or central location.
In order to avoid the problems inherent in the above mentioned conventional current-responsive line monitors, it is convenient to monitor on-hook/off-hook conditions by detecting the change of line voltage which, as mentioned previously, occurs when the telephone set goes off-hook. Since a line voltage detector monitors changes in the line voltage rather than the current flowing to each telephone set, a single monitor can detect on-hook/off-hook conditions on a given line no matter how many telephones are connected to the line.
For example, a telephone line monitor which detects changes in line voltage can be connected in a conventional PBX systems in a manner as shown in FIG. 6. FIG. 6 shows a PBX 5 with a telephone line 1 which is connected to a plurality of telephones 2 by means of jacks 4. In this system, voltage-sensitive line monitor 6 can be retrofitted to line 1 by simply plugging it into any jack 4 which is connected to the line 1. This jack may be located at or near PBX 5, and, thus, monitor circuitry 6 can be easily protected from user tampering and transfer of the detected signals can be accomplished by directly wiring the unit 6 to the PBX 5. Further, a single line monitor 6 can monitor all data such as off-hooks, dial pulses and tapping related to all telephone sets 2 connected to the lines 1. Consequently, the number of line monitors required in the system is equal to the number of lines rather than the number of telephones.
Despite the above-mentioned advantages, however, voltage-sensitive line monitors have generally not been used due to severe problems encountered in normal telephone systems. More particularly, the normal on-hook voltage in a telephone system depends upon the type of private branch exchange and the number of telephone sets connected to each telephone line and may vary over a wide range from line to line even within the same system. For example, a typical on-hook voltage may vary over a range from 12 to 105 volts. Thus, a voltage sensitive line monitor must be able to adapt to a large variation in voltage or several different models of monitors must be available to cover the voltage range.
Further, a voltage-sensitive line monitor detects an off hook condition be comparing the line voltage to an off hook threshold voltage. If the line voltage is below the threshold the telephone is deemed to be off-hook. If the line voltage is above the threshold the telephone is deemed to be on-hook. If the off-hook threshold voltage is preset to a fixed value during manufacture of the line monitor unit, it is not possible to reliably detect the off-hook due to the large variation in the line voltage from system to system. Even if the threshold is adjustable and is adjusted by the telephone system personnel at the time of installation, the line voltage on a particular line may change sufficiently with the connection of another telephone set to that line so as to require a readjustment of the threshold voltage.
Another problem with voltage-sensitive line monitors is that they are susceptible to interference by line noise. Such noise may occur due to power lines which are routed close to the telepone lines or other sources or may be due to nearby electrical equipment. The noise may cause instantaneous variations in the line voltage which are sufficent to cause improper operation of the line monitor if the off-hook threshold voltage is not set sufficiently low enough to accomodate such variations.
Accordingly, an object of the present invention is to provide a telephone line monitor which can detect on hook/off hook changes by monitoring telephone line voltage.
It is another object of the present invention to provide a voltage sensitive telephone line monitor that is capable of reliably detecting an off hook condition in telephone systems in which the line voltage varies over a wide range.
It is still another object of the present invention to provide a voltage sensitive telephone line monitor that is capable of reliably detecting an off-hook condition in telephone systems in which the line voltage may vary significantly due to noise.
It is a further object of the present invention to provide a voltage-sensitive telephone line monitor that does not require manual adjustment of the off-hook threshold voltage, either at the time of installation or thereafter.
It is a yet another object of the present invention to provide a voltage-sensitive telephone line monitor that can be easily retrofitted to existing telephone systems, including those which use plug and jack technology.
It is a still another object of the present invention to provide a voltage-sensitive telephone line monitor that allows each telephone line to be monitored with a single line monitor even tough a plurality of telephones may be connected to the line.